Head up displays (HUDs) used in automotive vehicles produce a virtual image by projecting a light pattern from an image source and reflecting it from a windshield in the line of sight of the operator. Windshields have a reflectivity less than 18%; thus in order to view the HUD image under bright sunlight conditions, a very bright image source is required.
The image sources used for HUDs include vacuum fluorescent displays (VFDs) and matrix addressable liquid crystal displays (LCDs). The brightness of a VFD is limited by the light emission mechanism of the phosphor which is bombarded by energetic electrons. Even though the brightness of VFDs have been improving over the years, it is expected to be less than 15,000 foot-lambert (fl) for the bluish-green color, and much less for other colors. On the other hand, LCDs can provide a much brighter colored reconfigurable image source if bright illumination is available. The conventional illumination source is an incandescent bulb with collimating optics in the proximity of the LCD. Since the image source and related optics are located beneath the vehicle dash board, the packaging space requirements and the heat output of the lamps near the image source often can not be accommodated, especially for high brightness lamps.